The present invention relates to an ink jet printer and its preliminary driving method and, in particular, to an ink jet printer for driving a head in advance in the case where printing is performed by discharging ink from the nozzle of the head, and to its preliminary driving method.
In a conventional ink jet printer in which ink is discharged from the nozzle of a head to print a letter (or to form an image), the ink is always in contact with air at the opening of the nozzle and hence gradually becomes dry, which inevitably produces a phenomenon in which the viscosity of the ink becomes higher.
Since the higher viscosity of the ink increases the surface tension of the ink, the ink is hard to discharge if a force to discharge the ink is the same as before. As a result, this changes the direction of discharging of the ink or produces variations in the speed of the drop of the ink to reduce the quality of an image to be produced.
Accordingly, an ink jet printer in which, in the case where ink is not discharged for a predetermined time, ink is discharged to a region not to be printed and then new ink is prepared in the opening of a nozzle to discharge the ink to a region to be printed has been known as a first conventional technology (see Japanese Patent Laid-Open No. 9-164694).
As a second conventional technology has been known a method of reducing the viscosity of ink by applying a driving waveform onto a head to the extent in which the ink is not discharged to sway the ink in the opening of the nozzle (see Japanese Patent Laid-Open No. 64-38246). As a third conventional technology has been known an ink jet printer having a first memory device for memorizing a plurality of preliminary discharging data to preliminarily drive a head and a second memory device for memorizing a specific preliminary discharging pattern data selected from the plurality of preliminary discharging data (see Japanese Patent Laid-Open No. 8-52885).
The ink jet printer of the above-mentioned third conventional technology performs a preliminary driving motion of discharging the ink from all nozzles at the same time to recover clogging in nozzles in low temperature surroundings. In this ink jet printer, in the case where a plurality of nozzles are divided into groups of a predetermined number of nozzles and each of the divided groups of nozzles is filled with the ink of different color, in order to prevent the inks from being mixed with each other, a preliminary driving motion is performed such that each ink is discharged a different number of dischargings according to the color of the ink and that the discharging motions of the inks of the plurality of colors are finished at the same time, or a preliminary discharging data is selected according to the condition of the head.
The above-mentioned conventional ink jet printers present a problem that because the preliminary driving motion is performed before the printer starts printing, there is a short open time before the printer starts printing actually and the ink becomes dry during the short open time to produce a considerable effect on the quality of printing.
The present invention has been made in view of the above circumstances, and it is the object of the present invention to provide an ink jet printer capable of shortening the time between a preliminary driving motion and a printing motion to improve the quality of printing and its preliminary driving method.
An ink jet printer in accordance with the present invention is an ink jet printer for performing printing by discharging ink from a plurality of nozzles mounted on a head moving with a carrier. The ink jet printer comprises a position detecting device, a preliminary waveform generating device, and a printing driving waveform generating device. The position detecting device detects the present position of the head. The preliminary waveform generating device compares the output of the position detecting device with a preliminary waveform starting position which is previously set to detect that the head reaches the preliminary waveform starting position. Then, the preliminary waveform generating device generates a preliminary waveform during the preliminary waveform driving region previously set in the period in which the head moves from the position where it is detected that the head reaches the preliminary waveform starting position to a printing starting position. The printing driving waveform generating device compares the output of the position detecting device with the printing starting position previously set just after the preliminary waveform driving region to detect that the head reaches the printing starting position. Then, The printing driving waveform generating device generates a printing driving waveform during a printing region previously set after the time when it is detected that the head reaches the printing starting position. The preliminary waveform sways ink in the opening of the nozzle of the head to the extent that the ink is not discharged from the opening of the nozzle and wherein the printing driving waveform discharges the ink from the opening of the nozzle of the head.
In the present invention, the preliminary waveform driving region is made in the period of movement of the head just after the head starts moving for printing until it reaches the printing starting position, and the preliminary waveform generated in the preliminary waveform driving region sways the ink in the opening of the nozzle of the head to the extent where the ink is not discharged to reduce the viscosity of the ink and the printing is performed in the usual printing region immediately after the head passes the preliminary waveform driving region. Accordingly, this can drive the head for printing before the viscosity of the ink, which is reduced by the preliminary driving motion, is increased.
Also, a method of preliminarily driving an ink jet printer in accordance with the present invention is a method of preliminarily driving the head of an ink jet printer just before ink is discharged from a plurality of nozzles mounted on the head moving with a carrier to perform printing. The method of preliminarily driving an ink jet printer in accordance with the present invention comprises a first step of detecting that the head reaches a preliminary waveform starting position previously set after the head starts moving to perform printing. The method further comprises a second step of generating a preliminary waveform during a preliminary waveform driving region previously set, in the period in which the head moves from the position where it is detected that the head reaches the preliminary waveform starting position to a printing starting position. The method further comprises a third step of swaying the ink in the head by the preliminary waveform to the extent that the ink is not discharged from the opening of the nozzle of the head.
In the present invention, the preliminary waveform driving region is made in the period of movement of the head just after the head starts moving for printing until it reaches the printing starting position. The preliminary waveform generated in the preliminary waveform driving region sways the ink in the opening of the nozzle of the head to the extent where the ink is not discharged, whereby the viscosity of the ink can be reduced just before the usual printing is started. Also, the present invention is characterized in that a printing driving waveform is inputted to the head instead of the preliminary waveform when the head reaches the printing starting position after the head passes the preliminary waveform driving region and that the head starts printing base on the printing driving waveform.